This invention relates in general to electrical submersible pumps and in particular to a restrictor for reducing downward flowing casing annulus well fluid during the initial start-up.
In a well, a static fluid level is established while the well is not being produced. This level is a function of the reservoir pressure at the well bore perforations. If this level is above the wellhead (ground level), it is a flowing well. If the level is below the wellhead, it is a dead well and requires artificial lift to flow.
FIG. 8 represents an example of an inflow performance relationship. It plots pressure at the perforations versus flow from the well. The pressure at the perforations could also be plotted as a fluid level (or fluid over the perforations ratio), as shown on the right scale of FIG. 8.
When an artificial lift system, such as an electrical submersible pump (ESP) is started, it adds pressure to the fluid so that it flows to the surface at a predicted flow rate. Before start-up of the ESP, the well bore is at a static condition with the well bore fluids stabilized in the well bore at a static fluid level. After the ESP is started and it has reached its design point, the well bore fluids are stabilized at a flowing fluid level. This drawdown follows the IPR curve in FIG. 8.
Between start and well bore stabilization, the fluid level is moving from the static level to the flowing level. This is called xe2x80x9cannulus drawdownxe2x80x9d. Therefore, the annulus volume has to be reduced or pulled down to its flowing fluid level. On start-up, almost all of the fluid being pumped is from the annulus above the pump intake, with only a small amount coming through the well bore perforations. As the annulus is drawn down, the flow from the annular volume decreases and the flow from the well bore perforations increases. The rate of this transfer is dependent upon the well annular volume (casing ID to tubing and equipment OD and the annular drawdown length) and the pumping flow rate.
At startup, the flow from the perforations upward past the motor to the pump intake will be zero or very low. The motor depends upon fluid flow by its skin to carry heat away. If this flow is too low, for too long a period, excessive heat can build up internally in the motor, causing damage or failure. This is especially true in wells which produce heavy, or viscous oil.
FIG. 9 shows graphically the heat rise in the motor, flow from perforations (flow by the motor), and annular flow to the surface versus time. In this example, the reduced cooling flow by the motor causes the motor to reach 480+ degrees F. in about 33 minutes. The drawdown to well bore stabilization takes over 583 minutes. In some wells, the transition time from start-up to steady state conditions may be as long as two days.